Recently I have tried taking the 4.5 hour core sleep at 8:00pm to 12:30am and the 90 minute nap at around 6am or 7am.
For some reason this has given me better results. I easily wake up from both sleeps, and better yet, I'm alert and ready to do work from 12:30am until my nap.
The normal biphasic sleep schedule (90 at 8pm, 4.5 at 2:30am) worked well most of the time but I often found myself in a terribly groggy state of energy from waking up from the 90m nap until my 4.5 sleep. Some people say that this is from being in a SWS state.
Currently, the thing that I have yet to fully master is the sleep cycle thing. While I give myself 90 minute cycles + 10 minute buffer, I still find myself waking at sporadic times -- sometimes the alarm wakes me up right in the middle of a dream, while just yesterday I woke up 30 minutes *before* the alarm! (But when I happen to wake up from just the right part of sleep, I'm gold and feel like I can conquer the world -- a feeling that lasts the entire day)
Can anyone point me to some resource (a scientific paper, preferably) that goes into detail with cycle lengths, how they vary (if at all) from night to night, and how certain factors -- a large meal beforehand, etc -- affect the cycle length? I'll search a bit myself and post if I find anything.